alpha scattering experiment model

• Structure of Atom
• Rutherford Atomic Model And Its Limitations

Rutherford Atomic Model and Limitations

Define rutherford atomic model.

Rutherford Atomic Model – The plum pudding model given by J. J. Thomson failed to explain certain experimental results associated with the atomic structure of elements. Ernest Rutherford, a British scientist conducted an experiment and based on the observations of this experiment, he explained the atomic structure of elements and proposed Rutherford’s Atomic Model.

Table of Contents

• Rutherfords Alpha Scattering Experiment

Observations of Rutherford’s Alpha Scattering Experiment

Rutherford atomic model, limitations of rutherford atomic model, recommended videos, frequently asked questions – faqs.

Rutherford’s Alpha Scattering Experiment

Rutherford conducted an experiment by bombarding a thin sheet of gold with α-particles and then studied the trajectory of these particles after their interaction with the gold foil.

Rutherford, in his experiment, directed high energy streams of α-particles from a radioactive source at a thin sheet (100 nm thickness) of gold. In order to study the deflection caused to the α-particles, he placed a fluorescent zinc sulphide screen around the thin gold foil. Rutherford made certain observations that contradicted Thomson’s atomic model .

The observations made by Rutherford led him to conclude that:

• A major fraction of the α-particles bombarded towards the gold sheet passed through the sheet without any deflection, and hence most of the space in an atom is empty .
• Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed . The positive charge in an atom is concentrated in a very small volume .
• Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180 o angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom .

Based on the above observations and conclusions, Rutherford proposed the atomic structure of elements. According to the Rutherford atomic model:

• The positive charge and most of the mass of an atom is concentrated in an extremely small volume. He called this region of the atom as a nucleus.
• Rutherford’s model proposed that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it with very high speed in circular paths. He named these circular paths as orbits.
• Electrons being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by a strong electrostatic force of attraction.

Although the Rutherford atomic model was based on experimental observations, it failed to explain certain things.

• Rutherford proposed that the electrons revolve around the nucleus in fixed paths called orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiations and hence an electron revolving around the nucleus should emit electromagnetic radiation. This radiation would carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Ultimately the electrons would collapse in the nucleus. Calculations have shown that as per the Rutherford model, an electron would collapse into the nucleus in less than 10 -8 seconds. So the Rutherford model was not in accordance with Maxwell’s theory and could not explain the stability of an atom .
• One of the drawbacks of the Rutherford model was also that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.
• Although the early atomic models were inaccurate and failed to explain certain experimental results, they formed the base  for future developments in the world of quantum mechanics .

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The Gold Foil Experiment

Structure of Atom Class 11 Chemistry

Drawbacks of Rutherford Atomic Model

What was the speciality of Rutherford’s atomic model?

Rutherford was the first to determine the presence of a nucleus in an atom. He bombarded α-particles on a gold sheet, which made him encounter the presence of positively charged specie inside the atom.

What is Rutherford’s atomic model?

Rutherford proposed the atomic structure of elements. He explained that a positively charged particle is present inside the atom, and most of the mass of an atom is concentrated over there. He also stated that negatively charged particles rotate around the nucleus, and there is an electrostatic force of attraction between them.

What are the limitations of Rutherford’s atomic model?

Rutherford failed to explain the arrangement of electrons in an atom. Like Maxwell, he was unable to explain the stability of the atom.

What kind of experiment did Rutherford’s perform?

Rutherford performed an alpha scattering experiment. He bombarded α-particles on a gold sheet and then studied the trajectory of these α-particles.

What was the primary observation of Rutherford’s atomic model?

Rutherford observed that a microscopic positively charged particle is present inside the atom, and most of the mass of an atom is concentrated over there.

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Rutherford Scattering Experiment

• Updated by Scienly
• On September 10, 2024

In this chapter, we will understand Rutherford’s scattering experiment and its observations and conclusions. In order to understand the arrangement of charged particles like electrons and protons in an atom, the British scientist, Ernest Rutherford and his co-workers in 1911 carried out a series of experiments using alpha particles known as Rutherford’s alpha scattering experiments. This experiment is based on the alpha particles (helium nuclei) experiment.

The Rutherford’s alpha particle scattering experiment is one of the most important experiments in the history of atomic physics. It laid the foundation for the modern understanding of the atom, challenging earlier models and introducing the concept of an atomic nucleus.

Before Rutherford’s experiment, J. J. Thomson had given the first model of an atom in 1904, also known as “plum pudding model of atom”. In this model, he proposed that an atom may be considered as a sphere of positively charge particles in which the negatively charged particles called electrons are embedded to make the atom as a whole neutral.

This model was discarded in 1911 on both theoretical and experimental due to the Rutherford’s atomic model. Now let’s understand Rutherford scattering experiment step by step.

Rutherford’s Alpha Particle Scattering Experiment

In this experiment, Ernest Rutherford bombarded a beam of alpha particles with a very thin gold foil. The thickness of gold foil is approximately 0.00006 cm. The alpha particles were emitted from the radioactive substances such as radium or polonium. An alpha particle is a doubly ionized helium atoms or ions with two units of positive charge, mass number is equal to 4 and missing two electrons. It is represented as +2 He 4 or +2 α 4 .

The pictorial representation of Rutherford’s scattering experiment is shown in the below figure.

From the above figure, it is clear that he produced a narrow beam of alpha particles from a radioactive substance like radium placed in a lead block and then passed it through a thin sheet of gold foil. A movable circular screen coated with Zinc Sulphide (ZnS) screen is placed around the gold foil in order to detect alpha particles after scattering.

When these alpha particles hit the screen, it produced a tiny flash of light (scintillation) on the screen, which could be observed through movable a microscope.

Observations from Alpha Particle Scattering Experiment

The following observations were made from the alpha particle scattering experiment:

• Most of the alpha particles (nearly 99%) passed through the gold foil with little to no deflection. In other words, most of the alpha particle moved in the straight path with no deflection. This means that most of the space inside the atom is empty, as shown in the above figure.
• Some of the alpha particles deflected with small angle, meaning that the positive charge of an atom occupies a tiny space.
• A very few alpha particles (approx. 1 in 20,000) deflected back through the deflection of 180 degrees. This means that the whole positive charge and mass of the atom is concentrated in a very small volume inside the atom.

Thomson’s atomic model could not explain these observations. According to Thomson’s model of atom, positive charge and mass inside an atom are uniformly distributed throughout its volume.

Conclusion from Rutherford Scattering Experiment

After performing a series of scattering experiments, Rutherford concluded that:

• Most of the space inside an atom is empty because most of the alpha particles passed through the gold foil undeflected.
• There must be the presence of a heavy positively charged mass at the center of an atom because some of the alpha particles are deflected by a certain angle.
• A heavy positively charged mass at the center of the atom is very small, which he named as nucleus.

On the basis of these observations, Sir Ernest Rutherford proposed the nuclear model of atom, which you will study in the next chapter.

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